Molecular packing and free volume in crosslinked epoxy networks

Abstract

A diglycidyl ether of bisphenol-A type difunctional epoxy resin was cured with different amounts of a tetrafunctional curing agent, namely metaphenylene diamine. The macrodensities of the cured samples were determined at room temperature and their volume expansion was measured from room temperature to 180°C using a dilatometer. By combining these data, specific volume-temperature plots were constructed. The occupied volume was determined either by extrapolating the specific volume to 0 K or by estimating the van der Waals volume for the network. The empty volume, packing coefficient and free volume fraction could then be calculated. It was observed that samples with a high degree of crosslinking showed good packing around 180°C but at the glass transition temperature ( T g) and in the glassy state, the packing was poor. It is postulated that after post-curing at 175°C, as the samples are allowed to cool, the constraints imposed by the crosslink affect the rate at which the samples contract; in samples with high crosslink density, the rate of contraction is subsequently low. Due to this, and also because of their high T g, a larger free volume is trapped in these samples when the microbrownian motion freezes at T g. With further cooling below T g the intersegmental separation is relatively higher in the highly crosslinked samples because the crosslinks do not provide a suitable environment for close packing. The likely effects of molecular packing on physical properties are briefly considered.